In present day electronic devices, continued scaling of integrated circuit features to smaller dimensions places continued pressure on patterning technology to define device features with required fidelity. Lithography is used to define patterns in mask material such as photoresist in order to provide a means to transfer desired patterns into permanent features of the device to be fabricated. For device critical dimensions (CD) smaller than about 50 nm, line edge roughness (LER) has emerged as a fundamental limiter to patterning devices.
Several features associated with lithographic patterning of photoresist contribute to LER and related problems. For one, the use of high molecular weight polymers has increased in recent technology generations because of smaller exposure doses required for patterning using such photoresists, resulting in higher substrate throughput. However, clustering that is inherent in high molecular weight polymers tends to increase LER in patterned high molecular weight photoresists, as opposed to lower molecular weight photoresists. LER is also exacerbated because of the fewer number of photons used to expose a photoresist material to be patterned, especially for high molecular weight photoresists. Each photon that strikes a photoresist material may induce photoacid diffusion into a spherical zone characteristic of the photoacid diffusion length. When only a small number of photons are used to expose a photoresist, as in high molecular weight photoresists, the overlapping spherical regions define rough boundaries at the edge of diffusion zones after the unexposed resist is removed. This so-called shot noise may define roughness on a scale that is a significant fraction of a CD for small photoresist features less than about 50 nm in CD.
The increased LER in patterned photoresist typically leads to increased LER in underlying substrate layers into which the photoresist pattern is subsequently transferred. In particular, during an etch step (pattern transfer) of an underlying layer, the LER from the photoresist is transferred at least in part to the material of the underlying layer being etched. While the etching used for the pattern transfer process, typically a reactive ion etch, may exhibit some capability of reducing LER, this is typically limited.
In the present day, reduction of LER remains an active area of efforts for most device manufacturers. Several additional processes have been tested to address this problem with marginal results. For example, dry chemical etch processes have the ability to remove material from the resist image but suffer pattern dependent loading effects from different exposed area to isolated to dense biases. In addition, it is important to maintain the photoresist CD within a tight tolerance. Hence it is desirable that any secondary applied technique to improve LER maintain the original photoresist attributes for profile, height, and CD. Dry chemical etch systems may also impart unwanted defects to the pattern resulting in yield loss. Another alternative approach that has been investigated in the use of a Deep Ultraviolet (DUV) cure where a rough pattern of photoresist lines or other small features is exposed to a lamp based platform in which heating through radiation exposure is used to smooth the photoresist lines. A drawback to this technique is a feature found in the corner of line segments where pattern pull back occurs, resulting in line deformation in such a way to render the device to be patterned useless.